1. Field of the Invention
The present invention is directed to a catalyst system for use with internal combustion engines to oxidize hydrocarbons, carbon monoxide and reduce nitrogen oxides in an exhaust gas when the engine is operated at both lean and stoichiometric air/fuel ratios. More particularly, the catalyst system of this invention includes two catalysts. One of the catalysts is designed specifically to store NOx emissions under lean conditions and reduce the stored NOx during rich operation. Additionally, this catalyst also serves to convert HC, CO and NOx during stoichiometric operation. This catalyst consists of two zones—a cerium-free zone containing aluminum oxide, oxides of alkali metals, alkaline earth metals and a high loading of precious metals, and a second zone of oxides of aluminum, alkali metals, alkaline earth metals, rare earth metals, or combinations thereof, and a lower loading of precious metals. The first catalyst can also be layered to achieve emission reduction under lean conditions.
The second catalyst is designed specifically to optimize the conversion of HC, CO and NOx under stoichiometric operations. The second catalyst also stores any NOx emitted from the first catalyst during lean operation and converts the stored NOx during the rich purges. The second catalyst contains precious metals, aluminum oxides, a high concentration of mixed oxides of zirconium and cerium and alkali metals or alkaline earth metals such as barium oxide or magnesium oxide.
2. Background Art
Catalysts have long been used in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into non-polluting gases including carbon dioxide, water, and nitrogen. When a gasoline powered engine is operated at a stoichiometric or slightly rich air/fuel ratio, i.e., between about 14.6 and 14.4, catalysts containing precious metals such as platinum, palladium and rhodium are able to efficiently convert all three gases simultaneously. Typically, such catalysts use a moderate loading of precious metals to achieve the high conversion efficiency required to meet the stringent emission standards of many countries. Because of the high cost of the precious metals, these catalysts are expensive to manufacture.
To improve vehicle fuel efficiency and lower CO2 emissions, it is preferable to operate an engine under lean conditions. Lean conditions are air/fuel mixtures greater than the stoichiometric mixture (an air/fuel mixture of 14.6), typically air/fuel mixtures greater than 15. While lean operation improves fuel economy, operating under lean conditions increases the difficulty of treating some polluting gases, especially NOx.
For some catalysts, if the air/fuel ratio is lean by even a small amount, NOx conversion is significantly reduced. One way to provide air/fuel control is through the use of a HEGO (Heated Exhaust Gas Oxygen) sensor to provide feedback to the control systems. HEGO sensors, however, over time can develop a lean bias as a result of poisoning. Accordingly, even with a HEGO sensor it is important to have a catalyst that can minimize noxious emissions, especially NOx, under lean conditions.
To decrease NOx emissions, under lean operating conditions, a lean NOx trap is frequently used. The NOx trap functions in a cyclic manner. The NOx trap stores NOx during lean operation. When the NOx trap approaches its NOx storage capacity, the engine is operated under rich conditions to reduce the stored NOx and purge the NOx trap. After the NOx trap has been purged, the engine can return to lean operation.
However, in addition to problems associated with thermal stability and sulfur tolerance, lean NOx traps have the following two known problems: (1) a problem referred to as “NOx release”, the release of unreduced NOx from the NOx trap during the transition from lean to rich conditions; and (2) a reduction in fuel economy that results from frequent purges of the NOx trap. FIG. 2 shows the NOx release for a catalyst with different oxygen storage capacities (“OSC”). This NOx release has been found to be greater than 35% of the total NOx emitted during a vehicle test using the FTP vehicle test cycle.
FIG. 2 also shows the effects of oxygen storage capacity of a lean NOx trap on the NOx release during the lean to rich transition. LNT L, which has the highest OSC, results in the largest amount of NOx release, while the lower the OSC (from LNT M down to LNT N), the lower the amount of NOx release. It is believed that the NOx release during the lean-rich transition is due to the exothermic heat generated from the oxidation of reductants, CO, HC, and H2, by the oxygen released from the oxygen storage material—the temperature rise can be as high as 80-100° C. If the bed temperature is higher than the peak storage temperature of the trap (i.e., in the range of decreasing NOx storage capacity) and the amount of NOx stored is near the capacity of the trap at that temperature, the exothermic temperature rise can cause the release of NOx in order to bring the amount of NOx storage back to the maximum amount that can be stored at the higher surface temperature. In FIG. 2, the trap with high oxygen storage capacity (OSC) had much larger NOx release than the trap with low oxygen storage capacity.
With regard to the fuel economy penalty, this is believed to be the result of high oxygen storage capacity (OSC), low NOx trapping capacity, and/or high exhaust flow rate in the lean NOx trap. The OSC requires additional reductants (i.e., fuel) to reduce the oxygen storage materials during each lean-to-rich transition, while the low NOx trapping capacity requires that the frequency of purges be increased.
To solve the above problems, the present invention provides a new catalyst system comprising two catalysts that can treat CO, HC and NOx, under both stoichiometric and lean conditions. Furthermore, the design of the present invention minimizes the purge NOx release and minimizes the fuel economy penalty associated with the rich purges.
The closest known prior art includes the following patents. For example, U.S. Pat. No. 4,024,706, incorporated by reference herein, teaches a method of enlarging the air/fuel ratio over which a catalyst operates by including an oxygen storage material. The method involves controlling the air/fuel ratio of the fuel mixture being burned by the engine such that the air/fuel ratio oscillates between a lean condition and a rich condition of equal magnitudes about the stoichiometric mixture.
U.S. Pat. No. 4,500,650 teaches a catalyst comprising a substrate, a refracting oxide layer, tungsten and/or one or more tungsten-containing compounds and one or more platinum group metals.
U.S. Pat. No. 4,678,770 teaches a method of creating a three-way catalyst, wherein rhodium is segregated from rare earth oxide to increase catalyst efficiencies under lean exhaust conditions.
U.S. Pat. No. 5,179,060 teaches a catalyst including a platinum group metal impregnation layer covering an alumina substrate layer. While this layering approach enhances the ability of the platinum to reduce exhaust emissions under stoichiometric conditions, it does not sufficiently reduce NOx emissions under prolonged lean engine conditions.
The present catalyst system solves the problem of the prior art by providing discrete catalyst compositions designed to maximize emissions reduction under stoichiometric conditions and lean conditions in a cost-effective manner.